The invention relates to a spark-erosion machining method and to a machine implementing:                a machining gap, situated between a tool electrode and a part, both conductors of electricity, a gap within which a multitude of erosive electrical discharges are generated, constituting a machining edge which is displaced according to the movements of the tool electrode,        a dielectric medium present within the machining gap,        said tool electrode being of cylindrical or tubular shape, moved along a number of axes of the machining machine relative to the machining edge and the part, x        a process regulator,        a numerical control system for controlling the displacements of the various axes of the machining machine,        a Nominal Trajectory of the tool electrode defined by means of a computer file supplied upstream of the numerical control system of the machining machine and consisting of a succession of linear segments defined relative to a part coordinate system X,Y,Z, this Nominal Trajectory describing a movement of the tool electrode relative to the part as it ought to proceed if no disturbance were to occur during the machining.        
In this type of machining method, the cylindrical tool electrode is moved along a trajectory substantially parallel to the surface of the part which is to be machined. The method uses a cylindrical tool electrode upon which movements are imposed in order to avoid short-circuits, counteract the disturbance inherent in the method, compensate for wear and keep the tool in the vicinity of and as close as possible to the trajectory.
Spark-erosion or electrical discharge machining is performed by means of an electrode used as a tool. The electrode can have a particular shape in the case of penetration spark-erosion. It may also be a wire stretched between two guides.
In the present document, a cylinder or a tube is considered that is used as the tool, the end of which is displaced along and/or in the vicinity of a predetermined machining path, which generally consists of a number of linear segments. The direction of advance of the tool may change from one segment to the next. The present invention deals more particularly with the latter case in which a tool of cylindrical or tubular shape machines by spark-erosion using its end. The machining progresses by successive layers; each layer more or less following the exact shape of the part to be machined, the latter serving as a guide for the tool. Said method is commonly called “spark-erosion milling”.
When machining parts using this method, because of the relatively high speeds of advance, there is a permanent risk of short circuits which stop the machining and can cause an abrupt collision between the tool and the part. When such a risk occurs, the tool must rapidly increase its separation from the part, more precisely widen its spark gap.
Commonly, these machining regulation movements are performed by forward or backward movements, and therefore by displacements of the electrode in the direction of the predetermined tool path. Such regulation movements are, in theory, possible also when finishing parts when the layer to be machined is very thin. The drawback in these working conditions however consists in that, to obtain a sufficient modification of the width of the gap, it will be necessary to perform considerably greater movements by moving backward on the predetermined tool path. Such movements of large amplitude cannot be performed fast enough because of the inertia of the elements of the machine tool supporting the tool; furthermore, in finishing operations, the width of the gap is not substantially modified by movements in the direction of advance on the main trajectory, considerably reducing the effectiveness of these movements.
The patent EP 0 555 818 discloses a similar machining method, notably characterized by the fact that the gap is situated in the same direction as the tool path, which poses stability problems when machining layers of small thickness with ongoing risks of collision that would be fatal to the tool. Also, the tool is subject to ongoing wear and must be brought closer to the part to be machined during its travel along the predetermined trajectory so as to compensate for said wear. The wear is compensated for by an ongoing downward advance of the Z axis of the machine and there is no provision for reorienting said advance during the travel of the tool.
The U.S. Pat. No. 5,438,178 describes a spark-erosion machining method using a wire electrode, in which the issue is to refine the lateral surface roughness of a part which did undergo a relative displacement from the position that it occupied during the roughing operation. Despite this displacement which is greater than the width of the machining gap, there is a desire to refine the surface roughness without having to reposition the part on the milling machine table.
The patent explains how the lateral surface of the part is to be used as a reference rather than forcing the tool (in this case an electrode wire) to precisely follow a predetermined trajectory. Deformation phenomena occur when the stresses internal to the material are released by the passage of the wire in direct cutting mode; to which are added the result of the thermal drifts. Consequently, it is no longer possible to define a precise trajectory with which to follow the outline of the part that has been preformed. Operation is similar to that of a measuring operation during which the tool would trace the surface of the part by following its outline. In the case explained by U.S. Pat. No. 5,438,178, the process regulator does not correct the position error of the tool in the direction of the normal to the surface.
This patent is representative of a typical issue in spark-erosion milling: in order to define a machining range, an ideal trajectory has to be programmed, bearing in mind that, for various reasons, the tool will not precisely follow said trajectory.
However, the patent U.S. Pat. No. 5,438,178 does not fully explain all the modalities according to which the tool must be distanced from said trajectory. Certain key aspects associated with spark-erosion milling cannot be deduced therefrom, notably:                The exact role of the planned trajectory which cannot be totally forgotten during the travel of the tool, despite the fact that it is not strictly observed.        The lateral surface of the part does not make it possible to deduce all the information needed to guide the tool during the finishing operation.        The process regulator(s) must incorporate specific functionalities to guide the tool off-trajectory, notably in order to provide corrections.        Phenomena other than the deformations of the part require the tool, during its travel, not to follow the planned trajectory. These are stresses imposed by the machining gap and the deformations of the electrode due to wear.        
The object of the invention is to develop a method designed to allow for the spark-erosion machining of parts, in particular their finishing, using cylindrical or tubular tools by successive layers, and to eliminate the machining risks and drawbacks mentioned hereinabove.